1. Brief Description of the Invention
The present invention relates to methods and systems for generating substantially dry atmospheric plasma which effectively can remove metal oxides, moisture and other undesirable compounds from metal and other surfaces just prior to packaging of electronic components.
2. Related Art
Preventing the failure of some integrated circuit (IC) packages due to the trapping of moisture or oxides between the package material, whether epoxy, BT Resin, polyamide, metal, or other material, and the IC. In addition, creating a clean activated surface in order to enhance the adhesion, wetability, flow, or bonding properties of the adhesive, molding compound, or metal bonding application. When any of these problems are not solved, the package can xe2x80x98popcornxe2x80x99 or crack during the application of reflow soldering which is done at high temperature ( greater than 180 C) or experience general problems in reliability for bonding. In addition, there also may be some benefit from treatment of the entire IC, leadframe, etc. prior to encapsulation. This can help activate the surface and remove any oxide, H2O, or other reducible compounds. This can prevent adhesion problems and encourage consistent spreading of the encapsulant material. Lastly, the advent of lead-free technology will also force this temperature to be higher during reflow and thus increase even further the stringent requirements to make a good package that won""t crack or fail.
By following a prescribed format for the use of a plasma system, treatment of the surfaces involved in the die attach processes can be improved. The basic steps involved are: complete deoxidization and treatment of a surface by use of a plasma process, including the activation of the surface while removing H2O and other reducible compounds prior to application of the glue, solder, or adhesion material to the surface whether plastic, metal, or other material. This is then followed by application of the glue, solder, or other material and subsequent bonding of the IC or package to the package or PCB respectively. In addition, the entire IC and lead frame structure can be treated prior to encapsulation in order to activate the surfaces and to remove oxides, H2O, or other reducible compounds. Exposure to the plasma just prior to the encapsulation step will remove impurities and improve wetability and flow as well as prevent delamination. Lastly, before making any metal to metal bonds, as in wire bonding for example, the plasma can be used to treat the surfaces in order to remove oxides, organic impurities, or other reducible material just prior to bonding. The advent of copper wire bonding will make this step more important so as to improve bond strength.
In accordance with the present invention systems and methods for forming an atmospheric, low temperature plasma greatly improves the IC packaging process by cleaning surfaces prior to encapsulation. This allows for strong plastic to metal and strong plastic to semiconductor material bonds to be formed and, when preferred, for successful copper to copper bond formation.
One aspect of the invention is a system for effecting bond region modification in electronic components comprising encapsulant materials, the system comprising: (a) a subsystem in which one or more electronic components are adapted to have an encapsulant applied and bonded thereto; (b) a source of initial gas; and (c) an electrical discharge apparatus connected to said source of initial gas for producing a primary gas from said initial gas, the primary gas comprising a plurality of unstable or excited species, the primary gas adapted to contact at least a portion of encapsulant bond regions of the electrical components in the sub-system and thereby effect encapsulant bond region modification prior to encapsulation.
Preferred are systems wherein the source of initial gas comprises a source of an inert gas, a reducing gas, and/or an oxidizing gas. Preferably, the initial gas has less than 1% water vapor, more preferably less than 500 ppm water vapor, and more preferably less than 50 ppm water vapor, and even more preferably less than 1 ppb water vapor. This limitation on the amount of water vapor in the initial gas helps to reduce, preferably eliminate, the tendency of the primary has to include species that might recombine to form water vapor after exposing the electronic component to primary gas.
Particularly preferred are those systems in accordance with the invention comprising means to maintain the primary gas mixture at a pressure ranging from about 0.1xc3x97105 Pa to about 3.0xc3x97105 Pa. Other preferred systems in accordance with the first aspect of the invention are those which include at least two electrical discharge apparatus, preferably in series relationship, wherein the apparatus employ same or different initial gas.
Also preferred are systems in accordance with the first aspect of the invention wherein two of the electrical discharge apparatus are in parallel, and either the same or different initial gas is employed. When two or more electrical discharge apparatus are employed, the apparatus may be same or different, and may use same or different power levels.
Other preferred systems in accordance with the first aspect are those which include a hood which encloses the conveying sub-system and primary gas, in preferably air-tight fashion.
A second aspect of the invention is a system for encapsulating one or more bonded metallic bond regions of a first electronic component bonded to one or more metallic bond regions of a second electronic component to form one or more metallic bonded regions, the system comprising: (a) a conveying sub-system upon which one or more metallically bonded, non-encapsulated electronic components travel from a metallic bonding station, electronic components having at least one metallic bond region; (b) a source of initial gas; (c) an electrical discharge apparatus for producing a primary gas from said initial gas, the primary gas comprising a plurality of unstable or excited species, the primary gas adapted to contact at least some of the regions of the electrical components adapted to be encapsulated and thereby effect encapsulant bond region modification; (d) an encapsulation station downstream of the bonding station, wherein the metallic bonded, non-encapsulated electrical components are encapsulated. Preferred systems in accordance with this aspect of the invention are systems wherein the encapsulation system is connected to the inerting station so as to avoid as much as possible the chance of moisture (water vapor and moisture are used interchangeably herein) adhering to or otherwise becoming a component of the non-encapsulated electronic component prior to encapsulation. Also preferred are systems in accordance with this aspect further comprising an annealing station connected to the bonding station and down stream of the bonding station.
Preferred are systems in accordance with this aspect wherein the electrical discharge apparatus comprises means to maintain the primary gas at a pressure ranging from about 0.1xc3x97105 Pa to about 3.0xc3x97105 Pa, such as a hood or tunnel, preferably air-tight. Preferred are systems comprising sources of inert, reducing, and oxidizing gases to make an initial gas mixture having at most 1% water vapor, preferably less than 500 ppm water vapor, more preferably less than 50 ppm water vapor, and even more preferably less than 1 ppb water vapor (volume basis).
Particularly preferred are those system embodiments of the first and second aspects of the invention equipped to de-oxide copper-containing metals and xe2x80x9cpurexe2x80x9d copper metal connections, as further discussed herein.
A third aspect of the invention is a method for encapsulating at least some metallic bonded regions in electronic components comprising metallic bonded regions, the method comprising transporting an electronic component having at least one metallic bonded region through a primary gas atmosphere comprising unstable or excited gaseous species, the gaseous species being substantially devoid of any electrical charges, the primary gas atmosphere having a pressure ranging from about 0.5xc3x97105 Pa to about 3.0xc3x97105 Pa, thereby forming a treated, bonded, non-encapsulated electronic component, and thereafter encapsulating at least some of the metallic bonded regions with an encapsulant material. Preferably, the primary gas is produced from an initial gas mixture comprising less than 1% water vapor, preferably less than 500 ppm water vapor, more preferably less than 50 ppm water vapor, and even more preferably less than 1 ppb water vapor. In one preferred method in accordance with the invention the initial gas comprises an inert gas and an oxidizing gas other than water vapor. In another method embodiment, the initial gas comprises a reducing gas and an oxidizing gas other than water vapor.
When used, the inert gas is selected from the group consisting of N2, Ar, He, Ne, Xe, Kr, and mixtures thereof; the reducing gas is selected from the group consisting of H2, CH4, NH3, and mixtures thereof; and the oxidizing gas is selected from the group consisting of O2, CO2, N2O, CO, NO, and mixtures thereof.
It is preferred that the primary gas is produced by flowing the initial gas through an electrical discharge apparatus as taught in U.S. Pat. Nos. 5,941,448 and 5,722,581, incorporated herein by reference. As disclosed in these patents, electrical discharge apparatus comprises first and second electrodes, a layer of dielectric material arranged on a surface of at least one of the first and second electrodes; the dielectric material faces one of the first and second electrodes, and the initial gas passes substantially transversely to the first and second electrodes.
Preferred electrical discharge apparatus operate at a power greater than or equal to 1 W/cm2, more preferably at a power greater than or equal to 10 W/cm2, and even more preferably at a power ranging from about 10 W/cm2 to about 100 W/cm2. All power values are normalized per unit surface area of the dielectric material.
The electronic component treated by the methods of the invention is preferably selected from the group consisting of an IC, a package into which an IC is to be placed, a package containing an IC and having terminals, a printed circuit board, a socket component for test and burn-in, a surface mount device, or combinations thereof.
Also preferred are methods in accordance with this aspect of the invention further comprising a step of transporting the treated electronic component to a packaging machine. Further preferred are methods in accordance with this aspect wherein the transporting is carried out in a substantially inert atmosphere.
A fourth aspect of the invention is a method of assembling an encapsulated electronic device, the encapsulated electronic device comprising at least one metallic bond between at least two previously unconnected metallic bonding regions, the method comprising the steps of:
(a) aligning, but not contacting a first component having a first metallic bonding region with a second component having a second metallic bonding region;
(b) exposing at least one of the first and second metallic bonding regions to a first primary gas, the first primary gas having a pressure ranging from about 0.1xc3x97105 Pa to about 3.0xc3x97105 Pa and comprising unstable or excited species and substantially devoid of electrical charges;
(c) contacting the first and second metallic bonding regions under conditions sufficient to form a metallurgical bond between the first and second metallic bonding regions thus forming a metallic bonded non-encapsulated electronic device;
(d) exposing at least a portion of the metallic bonded non-encapsulated electronic device to a second primary gas to form a metallic bonded, plasma treated non-encapsulated electronic device, the second primary gas having a pressure ranging from about 0.1xc3x97105 Pa to about 3.0xc3x97105 Pa and comprising unstable or excited species and substantially devoid of electrical charges; and
(e) encapsulating at least a portion of the metallic bonded, plasma treated non-encapsulated device.
Preferred are those methods in accordance with the fourth aspect wherein step (c) comprises a process selected from the group consisting of thermosonic bonding, ultrasonic bonding, compression bonding, wire bonding, solder bump bonding, or a combination of same; methods wherein step (a) comprises the use of a flexible dielectric interposer sheet; methods wherein both metallic bonding regions are exposed to the primary gas prior to step (c); methods wherein step (b) comprises generating the primary gas by passing an initial gas mixture through an electrical discharge apparatus; methods wherein step (b) comprises focusing a flowing stream of primary gas over the metallic bonding regions; methods wherein the first component is a dielectric interposer sheet having an area array of elongated, strip-like leads, and the second component is selected from the group consisting of a second dielectric sheet or a semiconductor wafer; methods wherein the first component is an integrated circuit chip and the second component is a TAB tape having one or more metallic leads thereon; and methods wherein the first component is a connection component having one or more leads having a frangible section, and the second component is a semiconductor chip.
A preferred atmospheric plasma process and system for use in the invention is the system designed by Air Liquide known under the trade designation ALIXSTAR, which effectively can remove metal oxides and other undesirable compounds from metal surfaces, and is one preferred system used just prior to packaging an IC in order to provide a clean surface on the metal bond sites of the IC and the package for metal to metal bonding. The treatment process of the invention also can achieve these results at atmospheric pressure which is something not currently available within the industry. The use of a method for treating metal surfaces just prior to bonding is becoming more important as copper to copper interconnect technology becomes the standard.
Further preferred embodiments and aspects will become apparent after review of the following description and claims.